In N,N-dimethylformamide (5 ml) were dissolved 6-(4-Amino-2-oxopyrrolidin-1-yl)-2H-1,4-benzoxazin-3(4H)-one (Reference Example 6; 148 mg, 0.60 mmol) and 4-(6-methoxy-3-oxopyrido[2,3-b]pyrazin-4(3H)-yl)butanal (148 mg, 0.60 mmol). Sodium sulfate (1.0 g) was added to this solution and the mixture was stirred overnight at room temperature. Sodium triacetoxyborohydride (508 mg, 2.40 mmol) was added to the reaction solution and the mixture was stirred at room temperature for 6 hours. After completion of the reaction, the reaction solution was diluted with dichloromethane, basified by adding a saturated sodium hydrogen carbonate aqueous solution, and the organic layer was then separated and washed in the order of saturated sodium chloride solution, water (3×) and saturated sodium chloride solution. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed under reduced pressure. The obtained residue was purified by silica gel column chromatography (chloroform/methanol) to yield 53 mg (18%) of the title compound in the form of a light yellow foamy solid.
